1. Field of the Invention
The present invention relates to a mold for a golf ball. More particularly, the present invention relates to a mold comprising a large number of projections for forming a dimple on a cavity surface thereof.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight to accelerate the transition of a turbulent flow over a boundary layer (which will be hereinafter referred to as a “dimple effect”). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in the flight distance of the golf ball. The acceleration of the transition of the turbulent flow increases a differentia between upper and lower separating points of the golf ball which is generated by a back spin. Consequently, a lift acting on the golf ball is increased.
Usually, the golf ball is formed by a mold comprising upper and lower portions having hemispherical cavities respectively. Assuming that the upper cavity is the Northern Hemisphere of a globe and the lower cavity is the Southern Hemisphere of the globe, the upper and lower portions are joined to each other over an equatorial plane (a plane including an equator). The inner peripheral surface of the mold is provided with a large number of projections. By the projections, dimples are formed on the surface of the golf ball. The shape of the dimple is obtained by inverting the shape of the projection.
A molding material (for example, a synthetic resin) leaks out from the parting line of the upper and lower portions. Therefore, a spew is generated in the equator portion on the surface of the golf ball. The spew is ground and removed by means of a grindstone. When the spew is generated in the dimple, it is removed with difficulty. The dimple is not formed on the equator in order to easily remove the spew. More specifically, a projection is not provided on the parting line of the portions. Consequently, a great circle which does not cross the dimple, that is, a great circle path is formed on the surface of the golf ball. When the great circle path is coincident with a portion having the highest peripheral speed of a back spin (which will be hereinafter referred to as the “highest speed portion”), the sufficient dimple effect cannot be obtained and a flight distance is insufficient. Furthermore, in the case in which the great circle path and the highest speed portion are coincident with each other and the case in which they are not coincident with each other, the different dimple effects are obtained. Therefore, the aerodynamic symmetry of the golf ball is deteriorated. In some cases, the dimple provided in the vicinity of the great circle path is slightly deformed during the removal of a spew. If the deformation is caused, the dimple effect becomes more insufficient when the great circle path and the highest speed portion are coincident with each other.
Japanese Laid-Open Patent Publications Nos. Hei 6-143349 and Hei 11-137727 have disclosed a mold in which a pin for forming a dimple is provided on a parting surface. An arcuate spew is generated along a dimple in a golf ball formed by the mold. It is hard to remove the spew.
Japanese Laid-Open Patent Publication No. Hei 9-28833 has disclosed a mold in which a parting surface is formed by a plurality of inclined surfaces. In the mold, a perpendicular portion to a mold clamping direction is not present on the parting surface. In the mold, the parting surface is easily deformed by a mold clamping pressure.
Japanese Laid-Open Patent Publications Nos. Sho 62-47379 and Hei 8-173576 have disclosed a mold in which a parting line is concavo-convex shaped. In a golf ball obtained by the mold, a plurality of dimples are arranged in a first line of a seam corresponding to a parting line. Therefore, a sufficient dimple effect cannot be produced when the highest speed portion of a back spin is coincident with the seam.
Thus, all conventional molds for forming a golf ball in which a dimple is present on the equator have some defects. It is an object of the present invention to provide a mold which can obtain a golf ball having an excellent aerodynamic characteristic and is excellent in a durability.